Abstract [en]

Top predators in several aquatic food webs regularly display elevated reproductive failure, caused by thiamin(vitamin B1)deficiency. The reasons for these low-thiamin levels are not understood and information about the transfer of thiamin from the producers (bacteria and phytoplankton) to higher trophic levels is limited. One main concern is whether cyanobacterial blooms could negatively affect thiamin transfer in aquatic systems. Laboratory experiments with Baltic Sea plankton communities and single phytoplankton species were used to study the effect of filamentous cyanobacteria on the transfer of thiamin from phytoplankton to zooplankton. Experiments showed that the thiamin content in copepods was reduced when exposed to elevated levels of cyanobacteria, although filamentous cyanobacteria had higher levels of thiamin than any other analyzed phytoplankton species. Filamentous cyanobacteria also had a negative effect on copepod egg production despite high concentrations of non-cyanobacterial food. Phytoplankton species composition affected overall thiamin concentration with relatively more thiamin available for transfer when the relative abundance of Dinophyceae was higher. Finally, phytoplankton thiamin levels were lower when copepods were abundant, indicating that grazers affect thiamin levels in phytoplankton community, likely by selective feeding. Overall, high levels of thiamin in phytoplankton communities are not reflected in the copepod community. We conclude that presence of filamentous cyanobacteria during summer potentially reduces the transfer of thiamin to higher trophic levels by negatively affecting phytoplankton and copepod thiamin content as well as copepod reproduction, thereby lowering the absolute capacity of the food web to transfer thiamin through copepods to higher trophic levels.

In thesis

Fridolfsson, Emil

Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.

2019 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

Thiamin (vitamin B1) is required for several life-sustaining processes in most organisms and cells, e.g. in the conversion of food to energy. It also serves as an antioxidant and is important for proper nerve signaling. Thiamin is produced predominantly by bacteria and phytoplankton in the aquatic food web. Not all bacteria and phytoplankton, nor any organisms in higher trophic levels can produce thiamin; instead, they rely on a continuous external supply and uptake of this essential compound. Thiamin deficiencies occur episodically in a wide range of taxa, especially in higher trophic levels. In the Baltic Sea, thiamin deficiencies has been most pronounced in salmon (Salmo salar) and recently deficiencies are also reported for other fish species and birds.

This thesis focuses on thiamin dynamics in lower trophic levels, covering primary producers as well as primary consumers, as this topic has not received much attention previously. Thiamin content of common phytoplankton and zooplankton species and the transfer between them was investigated in experiments and field studies. The relationship between thiamin deficiency and underlying environmental factors was also investigated using monitoring data.

Thiamin content differed among phytoplankton classes, species and even strains. Filamentous Cyanophyceae had considerably higher thiamin content than other classes. However, thiamin transfer to copepods was lower, probably associated with difficulties ingesting the filaments. Moreover, thiamin content in seston varied seasonally, being highest during summer when both Prymnesiophyceae and filamentous Cyanophyceae were more abundant. Thiamin content in the two size fractions correlated strongly and was always higher in the smaller size fraction, illustrating the importance of picoplankton and bacteria in the food web. Also, seston thiamin content was higher in the Baltic Proper than in the Skagerrak. Copepods differed in thiamin content among genera, as well as between locations. Acartia sp. had the highest thiamin content and copepods from the Skagerrak had higher levels than congeners from the Baltic Sea. Thiamin deficiency syndromes in salmon was associated with changes in all trophic levels, as well as changes in several abiotic variables.

In all, this thesis provides new insights on thiamin content and seasonal dynamics in various phytoplankton and zooplankton species, the transfer of this vitamin between trophic levels as well as the overall importance of thiamin in the aquatic food web.